The present invention relates to display devices, and, more specifically, the invention relates to display devices, such as an active matrix type of liquid crystal display device.
An active matrix type of liquid crystal display device includes a plurality of gate signal lines disposed so as to extend in the x direction and to be juxtaposed in the y direction and a plurality of drain signal lines disposed so as to extend in the y direction and to be juxtaposed in the x direction; and, these gate and drain signal lines are formed on a liquid-crystal-side surface of one of a pair of substrates disposed in opposition to each other with a liquid crystal interposed therebetween. Areas each surrounded by adjacent ones of the gate signal lines and adjacent ones of the drain signal lines are formed as pixel areas, and an array of these pixel areas is constructed as a liquid crystal display part.
Each of the pixel areas includes a thin film transistor to be driven by a scanning signal received from a gate signal line formed on one side of the pixel area, and a pixel electrode to be supplied with a video signal via this thin film transistor from a drain signal line formed on another side of the pixel area.
This pixel electrode causes an electric field to be generated between the pixel electrode and a counter electrode formed on a liquid-crystal-side surface of either of the substrates, and the optical transmissivity of the liquid crystal of the corresponding pixel areas is controlled by the intensity of the electric field.
The above-mentioned signal lines, thin film transistors, electrodes and the like are micromachined by selectively etching stacked conductive layers, an insulating layer, a semiconductor layer or the like by so-called photolithography techniques.
In this case, it is common practice that the conductive layers formed as different layers separated by the insulating film are electrically connected to each other through a contact hole formed in the insulating film. However, if the insulating film is made of multiple layers, it is difficult to form the sidewall of the contact hole into a smooth shape, because of the difference in etching rate, film thickness or the like between the multiple layers, and techniques to overcome this difficulty need to be introduced. This is because, in the case where a connection is provided between the conductive layers through the contact hole, a disconnection is easily created in a conductive layer formed to reach the sidewall.
As one example, there is a known technique which is applicable to a thin film transistor having a so-called top gate structure in which tapering of contact holes is performed for forming electrodes of the thin film transistor, by wet etching, when the contact holes are formed in a structure in which a silicon oxide film, a silicon nitride film and a silicon oxide film are stacked in the named order (refer to Patent Document 1).
As another example, there is a known technique in which the materials of multiple insulating films which form a contact hole are individually selected, so that the etching rates of the respective insulating films increase stepwise or continuously from the bottom layer toward the top layer (refer to Patent Document 2).
As yet another example, there is a known technique in which contact holes are formed in a stacked structure made of a silicon oxide film and a silicon nitride film through one etching process (refer to Patent Document 3).
Patent Document 1 is JP-A-11-111990.
Patent Document 2 is JP-A-9-251996.
Patent Document 3 is JP-A-11-258634.